A typical disc drive includes one or more discs mounted for rotation on a hub or spindle. A typical disc drive also includes one or more transducers supported by a hydrodynamic air bearing which flies above each disc. The transducers and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls a disc drive to retrieve information from the discs and to store information on the discs.
An actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations. A servo controller samples the position of the data heads relative to some reference point and generates an error signal based upon the difference between the actual position and the reference position. This error signal is then used to drive the data head to the desired reference point, typically by demanding a current through a voice coil motor (VCM) which forms part of the actuator.
Information is typically stored on the discs by providing a write signal to the data head to encode flux reversals on the surface of the disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the actuator so that the data head flies above the disc, sensing the flux reversals on the disc, and generating a read signal based on those flux reversals. The read signal is then decoded by the drive controller to recover the data represented by flux reversals stored on the disc, and consequently represented in the read signal provided by the data head.
Thus, a disk drive mechanical structure is composed of multiple mechanical components that are pieced together to form the final disk drive assembly. Each of these components has various resonant modes that if excited by an external energy source will cause the part to physically move at the natural frequencies of oscillation for the component in question. This movement can occur in a bending mode, a twisting mode or a combination of the two. If the component is highly undamped (i.e. the resonance is high amplitude, narrow frequency band) it will tend to oscillate with a minimal external driving energy. This oscillation results in physical motion of the data head, causing off track errors and potential fly height problems. These oscillations are often referred to as "resonances."
If resonances occur in a disk drive, they can severely limit drive performance, both in seek mode and track-follow mode. To obtain the optimal disk drive performance requires that there be no resonances present. However, this scenario is not physically possible. Every mechanical component has a natural frequency of oscillation. Nevertheless, it is desirable to reduce or minimize the resonances. One way of doing this is to mechanically damp the mechanical components and thereby decrease the amplitude of the resonant mode. This can be done by careful design, the end result being a reduction in the amplitude of the oscillation to a level that is deemed acceptable to achieve a desired drive performance.
However, there are situations where a component is not able to be mechanically damped. This could occur, for example, because of materials used or because of design time constraints. When this scenario occurs, the only way to improve drive performance is to make sure that no excitation energy at the natural frequency of oscillation reaches the mechanical component to start it oscillating. The present invention concentrates on this approach.
As mentioned above, typical disc drives demand a current through a voice coil motor (VCM) to drive the data head to the desired position. When a frequency spectrum of demand current is analyzed it is found that the spectrum is composed of frequency components from direct current (DC) all the way up to multiple kilohertz (kHz). If VCM current is driving the actuator at the same frequency as the natural frequency of a mechanical resonant mode of a mechanical component, the energy may be sufficient to excite the mechanical structure into oscillation. This is very undesirable and will at least degrade disk drive performance or at worst will cause the servo system to go unstable.
The method employed by servo engineers to minimize the chances of the mechanics oscillating is to use hardware electronic filtering and/or digital filtering of the VCM current via a microprocessor or digital signal processor. Both types of filters achieve the same overall result They reduce the driving force energy (i.e. the current flowing) at frequencies deemed a concern.
One type of filter that is widely used to remove driving energy at the mechanical resonant modes is known as a notch filter. A notch filter is a band-rejection filter that produces a sharp notch in the frequency response curve of the disc drive. When a notch filter is activated by the servo control loop, the open loop response ends up a summation of the original response plus the notch filter response. If the notch filter is centered about the frequency where the peak amplitude of the mechanical resonance occurs, then the driving force energy at this frequency can be reduced so that there will be little or no energy made available to excite the mechanical structure.
The problem with the notch filter, however, is that if the center frequency of the mechanical resonance does not align with the center frequency of the notch filter then the attenuation of the driving current may not be enough to keep the structure from going into oscillation. This will occur if the mechanical resonance has shifted in frequency. This can easily occur on a drive to drive basis or even from one data head to another.
Present disk drives have fixed notch filters that are designed to cover a spread in mechanics. Such a filter, for example, is described in U.S. Pat. No. 5,032,776. Such filters remove driving energy at frequencies which would not cause the mechanical structure to oscillate for a given head or for a given drive. Thus, they are not optimal solutions. Furthermore, such filters cannot guarantee that the gain of the resonance will remain below 0 dB.
Methods also exist to implement adaptive filtering techniques by implementing digital signal processing algorithms in the servo controller. Such a method, for example, is described in U.S. Pat. No. 5,325,247. Such methods involve complex microcontroller code and are heavy on computational time. Furthermore, such methods cannot also guarantee optimal results under all circumstances.
As disk drive servo systems continually require higher open loop bandwidths to track follow accurately, the requirement for improved filtering techniques increases also. The present invention provides an economical means of providing a high degree of attenuation of the mechanical resonance frequencies and offers other advantages over the prior art.